Impact of ozone pollution on gross primary productivity during a large-scale emission reduction: Implications for strengthening carbon sink over megacity clusters in China.

Ozone (O3) pollution poses a significant threat to plant growth and diminishes ecosystem carbon sequestration capacity in China, representing a critical challenge to achieving "dual carbon" goals. A large-scale emission reduction that occurred during the Coronavirus disease 2019 lockdown offered a great opportunity to understand the complex mechanisms behind the impact of meteorology and emissions on gross primary productivity damage (GPPd). Hence, a state-of-the-art air quality model was used to assess the underlying linkages between O3, relative humidity (RH), and GPPd during the lockdown period, and to calculate the contribution of individual physical processes and chemical reaction on GPPd levels across six megacity clusters in China. The results showed that RH was more important in dry climate areas, which enhanced GPPd by 2%-4%, whereas MDA8 O3 enhanced GPPd by 3%-4% in humid areas. A strong relationship observed between O3 and GPPd variations and the O3 precursor sensitivity (OPS) depicted that transferring the OPS to the NOx-limited regime is a prerequisite for achieving synergistic benefits in O3 pollution reduction and GPPd mitigation. By tracking GPPd change response to the emission reduction, GPPd can be mitigated to varying degrees in six megacity clusters as anthropogenic emission reduction measures are implemented, especially across the tipping point of 40%. This study has important implications for achieving synergistic benefits in pollution reduction and carbon emission mitigation and highlights the importance of implementing sustained and deep NOx emission control measures to transfer OPS to the NOx-limited regime under carbon peaking and carbon neutrality vision.